This invention relates to the purification of saturated fluorohalocarbons and fluorohalohydrocarbons containing unsaturated impurities, and in particular to an oxidative process for removing unsaturated impurities from the hydrogen-containing members of said saturated halocarbons having 2 to 6 atoms.
Saturated fluorohalocarbons such as chlorofluorocarbons having 2 to 6 carbon atoms are, when pure, inert and non-toxic gases and liquids at ordinary temperatures and pressures, and are useful, among other applications, as refrigerants, propellants, blowing agents, solvents and intermediates for the production of other halogenated products.
Hydrogen-containing fluorocarbons and chlorofluorocarbons, are currently of interest as replacements for many of the commercially employed perchlorofluorocarbons because of their greatly reduced ozone depletion potentials.
In the manufacture of the saturated fluorohalocarbons and fluorohalohydrocarbons, undesirable unsaturated by-products are often also formed and contaminate the desired products. The unsaturated compounds are highly objectionable as contaminants as they are often toxic, and for most uses their concentrations in the saturated product must be lowered to innocuous levels of 10 ppm or less. Distillation and other conventional physical methods which may be used for their removal to acceptable levels are, however, generally ineffective or too costly. Accordingly, various chemical treatments have been heretofore proposed for this purpose; for example:
Heberling, U.S. Pat. No. 2,999,855 (1961) discloses that unsaturated fluorocarbons and saturated fluorohydrocarbons can be removed from saturated perfluorocarbon product streams by treatment with aqueous alkaline potassium permanganate at 20.degree. to 95.degree. C.
Weeks, U.S. Pat. No. 3,696,156 (1972) proposes to remove unsaturates from saturated perfluorohalocarbons by contacting the impure material in the vapor phase at 180.degree. to 250.degree. C. with alumina containing an alkali metal or alkaline earth metal hydroxide.
Bell, U.S. Pat. No. 4,129,603 (1978) discloses that the content of by-product 1,1-di fluoro-2-chloroethylene, CF.sub.2 .dbd.CHCl, in 1,1,1,2-tetrafluoroethane, CF.sub.3 CH.sub.2 F, produced by vapor phase hydrofluorination of a 1,1,1-trihalo-2-chloroethane over a chromium oxide catalyst at 300.degree.-400.degree. C., can be reduced by contacting the impure product with aqueous alkaline metal permanganate at 10.degree. to 40.degree. C. There is no disclosure as to possible loss of some of the fluorohydrocarbon, CF.sub.3 CH.sub.2 F, during the alkaline permanganate treatment in view of the above Heberling disclosure.
On the other hand, Potter, U.S. Pat. No. 4,158,675 (1979) teaches the removal of 1,1-difluoro-2-chloroethylene from the 1,1,1,2-tetrafluoroethane product stream produced as in Bell above by vapor phase hydrofluorination of a 1,1,1-trihalo-2-chloroethane, by passing the impure stream together with hydrogen fluoride over a chromium oxide catalyst at lower temperatures (100.degree.-275.degree. C.) than the hydrofluorination temperatures (300.degree.-400.degree. C.) of the tetrafluoroethane production step.
None of these prior processes is entirely satisfactory from a commercial standpoint. The aqueous alkaline metal permanganate treatments of the Heberling and Bell patents, for example, require that the halocarbon products exiting the treatment medium be dried (separated from its entrained water) before further refining, which adds to the expense of the treatment. Moreover, where saturated halohydrocarbon products are being treated, the possibility exists that some of the valuable saturated material could be lost to the alkaline oxidative medium along with the unsaturated impurities.
On the other hand, the high temperatures of the Weeks and Potter treatments are objectionable because the higher temperatures increase the cost of removing the unsaturated impurities. Further, the Weeks treatment appears limited to perhalocarbons inasmuch as hydrogen-bearing halohydrocarbons are possibly susceptible to dehydrohalogenation to form unsaturated products under the high temperature alkaline conditions of the disclosed process.
Frazer and Scalione, U.S. Pat. No. 1,345,323 (1922) describe the preparation and use of certain amorphous metal oxides as catalysts suitable for the oxidation of readily oxidizable gases, for example, carbon monoxide, ammonia, sulfur dioxide, aldehydes, alcohols and toluene, by passing them mixed with oxygen or air through the catalyst at ordinary or only slightly elevated temperatures. Included among the metal oxides are cupric oxide, silver oxide, cobaltic oxide, manganese dioxide and intimate mixtures of the metal oxides called hopcalites, such as, for example, CuO--MnO.sub.2, Ag.sub.2 O--MnO.sub.2, Co.sub.2 O.sub.3 --MnO.sub.2 and CuO--Ag.sub.2 O--Co.sub.2 O.sub.3 --MnO.sub.2.
A later publication, Johnson and Gammon, U.S. Naval Research Laboratory Report NRL 6582, dated Jul. 20, 1967, describes a study of the decomposition of methylchloroform, vinylidene chloride, trichloroethylene or tetrachloroethylene when exposed in humid air at temperatures up to 300.degree. C. to a hopcalite catalyst that is a co-precipitate of cupric oxide and manganese dioxide. The percentage decomposition of the chlorocarbons increased with increasing temperature. At 300.degree. C., it ranged from 70% for tetrachloroethylene to 100% for methylchloroform. All the chlorocarbons yielded by-product hydrogen chloride under the decomposition conditions.
Musick and Williams, Ind. Eng. Chem., Prod. Res. Develop, Vol. 13, No. 3, 1974, pages 175-179, describes a broader follow-up study involving 19 halogenated hydrocarbons. The halocarbons, mixed with humid air, were each exposed to a hopcalite catalyst based on cupric oxide and manganese dioxide in a catalytic burner operated at 305.degree. and 315.degree. C. Seven of the compounds, all saturated perhalocarbons having high fluorine contents, showed no detectable loss at 305.degree. or 315.degree. C. Another 9 saturated compounds, including the halohydrocarbons chloroform, dichlorofluoromethane, chlorodifluoromethane, methyl chloroform and 1,1,1-trifluoro-2-chloroethane, suffered significant decomposition losses under the same oxidative conditions. The remaining 3 compounds, all unsaturated, namely vinylidene chloride, trichloroethylene and tetrachloroethylene, were extensively decomposed. Hydrogen halide was again detected in the burner effluents of all the halocarbons undergoing decomposition loss. The presence of hydrogen halide is objectionable since additional processing is needed to remove it.
It is an object of this invention to provide a new and effective oxidative process for reducing the concentration of unsaturated impurities in fluorohalocarbons and fluorohalohydrocarbons, in particular in such halocarbons having 2 to 6 carbon atoms and where halo represents one or more fluoro, chloro and/or bromo groups.
Another object is to provide a process as above that operates at relatively low temperatures.
Still another object is to provide a process as above that reduces the unsaturated impurity content of fluorohydrocarbon and chlorofluorohydrocarbon product streams substantially without yield loss of the hydrogen-bearing halocarbon components.
A further object is to provide a process as above that is effective in lowering the unsaturated impurity content of the saturated product streams to low levels without the coproduction of hydrogen halides, thereby facilitating the direct recovery of the saturated products.